Mobile telephones are widely used for the convenience of being able to communicate at a variety of locations. FIG. 1 depicts a typical prior art mobile telephone communication system 100. The system comprises a mobile telephone 110, which transmits an uplink radio signal 111 to a base station 120. The base station 120 transmits a downlink radio signal 112 to the mobile telephone. The base station 120 is also connected to telephone equipment 130 through a connection 122 which is usually part of the public switched telephone network (PSTN). The telephone equipment 130 can be any type of telephone, including another mobile telephone or an automated answering unit or a modem connected to a computer. The system 100 provides a point-to-point two-way connection between the mobile telephone 110 and the telephone equipment 130.
As a public service, the need to locate and identify mobile telephones is well known. Much attention has been devoted to a recent FCC requirement that a mobile telephone making an emergency call be located, regardless of where the telephone is in the wireless service area, see FCC E911 Rulemaking Docket No. 94-102. A mobile phone can be located by monitoring its uplink radio transmissions by triangulation or other methods well known in the prior art.
For private services, it would also be useful to be able to locate and identify mobile telephones in the vicinity of a service or goods provider so that mobile telephone initiated transactions can be completed with a high degree of reliability and that are resistant to spoofing and fraud. A typical example of a useful transaction is the execution of a payment using a mobile telephone instead of a credit card.
It is well known that a signal proportional to the envelope of a transmitted radio signal transmitted can be detected, see U.S. Pat. No. 5,649,296 incorporated herein by reference. This signal is commonly called the baseband envelope. Of special interest to the invention are signals transmitted according widely used air interface standards such as global system for mobile communication (GSM) telephones. GSM services include GSM1800, which operates in the 1800 MHz frequency band, also referred to as PCN or PCN 1800, or DCS 1800. GSM1900 operates in the 1900 MHz frequency band, also referred to as PCS 1900 and DCS 1900. GSM900 operates in the 900 MHz frequency band. Also of interest are signals transmitted by mostly Japanese personal digital communication (PDC) devices.
It is desired to use the baseband envelope of signals transmitted by mobile telephones for radio frequency identification (RFID). More specifically, it is desired to identify, authenticate and locate mobile telephones without having to decode the underlying encrypted wireless signal. Furthermore, it would be advantageous to provide for mobile telephone identification without having to modify existing mobile telephones, wireless networks, and wireless communications air interface standards.
It is well known that the envelope of a radio signal can be used to convey information. This is the well-know amplitude modulation (AM) technique. It is understood that the instantaneous amplitude of a radio signal is the same as its envelope. U.S. Pat. No. 6,072,792 issued to Mazur et al. describes a power control apparatus for a TDMA transmitter. The technique described therein is used to reduce interference in the transmitted signal. This allows a receiver to extract the TDMA signal from adjacent time slots.
It is desired to deliberately manipulate the envelope transmitted by a mobile telephone so as to convey information that can be used to achieve the aforementioned RFID without having to modify existing mobile telephones, wireless networks, communications standards, and air interface standards for mobile telephones.